Methods and apparatus for detecting hazardous line voltages known in the prior art involve connecting fusing resistors and threshold sensing elements into the two-wire line, overvoltages then causing rupture or short-circuiting. Such protection in an electronic telephony installation is illustrated in Swedish Patent Application No. 7600255-9, for example. Protection here comprises fusing resistors and Zener diode combinations for preventing overvoltages arising between the line wires and ground. Furthermore, it is already known to connect test relays to both wires, e.g. in the vicinity of the hybrid coupler. When the line condition is to be tested, the relay opens the line and its impedance is measured, whereby information is given as to possible abnormal conditions such as short-circuiting or rupture.